PapaBearXJ88

I hope someone can help make heads or tails of this.
I parked my 1988 Renix XJ at 4:00pm and it was working fine. When I got in at 11:30pm to go home, it would not start out in 1st gear, felt like it was starting in 3rd. I manually shifted and it worked fine. There were no problems with shifting beforehand or any indication that the transmission may be going out.

I started with checking the fuses, then used a jump wire to beef up the ground. I went through Renix54's Mostly Renix tips and refreshed the trans plug and the ECU connector (I disconnected and inspected, but haven't sprayed with contact cleaner yet, just got some today and plan to redo with contact cleaner and dielectric grease).

I tested the TPS by back-probing the connector (after cleaning the throttle body) the voltage supply was 4.64 volts, the TPS output was .81 volts closed, 4.46 volts at full-throttle.

I tested the TPS at the TCU as well (D1 TPS Voltage Supply, D2 TPS Input, D3 TPS Ground). Voltage supply was at 4.6 volts, Input was at 3.55 volts (at closed throttle), and 1.04 volts at full-open throttle (don't understand why the high-low values seem to be reversed from the test at the sensor?)

These values are a little off, but are they off enough to make the TPS cause the trans not to shift into 1st gear on take-off?

I swapped out the ECU with one that a friend had to eliminate the ECU - no change, same issue.

Two Notes about the TPS:
1) The TPS test at the TCU was taken after I had made an attempt to adjust the sensor. There was no change in the way the engine runs (no change at all) or in the shift issue after the adjustment.
2) For quite a while, I had had a problem with the engine revving high upon start-up (2500-3000 rpm). It could take several attempts to get it to start at a normal idle. Since this shifting issue, the engine starts at a normal idle - no revving. I had been told that the TPS could cause this, could this be an indication that it is the TPS as well?

I am running out of other options, but I am wondering if it is time to break down and replace the TPS to rule it out? (they're rather expensive, didn't want to replace it if it isnt the problem).

I printed a Renix TCU pinout photo that I got from another post and took a pic of my TCU connector that I used as a key and have attached the photo. (I'm also attaching the C-side of the connector in case anyone else can use it for reference)

Any ideas?

67 GMC

There's an inline fuse and a resistor on the way to the TCU that's kind of buried in the harness(so check that too). Does it drive the same way with the TCU disconnected or is there a noticeable change with it connected. If the speed sensor is bad, it usually drives only in 1st (won't shift out since it doesn't think the jeep is moving). TPS can be checked by removal and testing OHMS as you move it. DIPSTICK ground should be really, really clean and all wires to it checked for bad crimps, connections etc. I remade that whole mess of wire to avoid intermittent issues.

PapaBearXJ88

Thanks for the quick response 67 GMC. I checked the fuse going to the TCU, but didnt know about the resistor, I'll check that too.

It drives exactly the same with TCU unplugged as it does with it plugged in. It drives only in 3rd. Only time I get into any other gear is if I manually shift.

I will clean, inspect and rework the dipstick ground wires as well to see where that gets me. Thanks again.

67 GMC

Not sure if you have this already but good information.

http://jeep.zerok.ru/index.php?page=52

PapaBearXJ88

thanks 67GMC, I did have that - great site, lots of useful info, use it often.

I have now refreshed all grounds in the engine compartment, think I have narrowed it down to TPS or TCU. I am thinking it would be more likely that it would be the TPS than TCU, so I'm going to change TPS in the morning.

Mine tested close, but was slightly off, and I didnt have an analog multimeter, so even though the values were good on reference voltage, good on low voltage, a little low on the other. meter showed variable voltage from closed to open, can't say for sure that it was a smooth transition, but I reckon with it being at least 15 years old (the time that I've owned it) and likely much older, it can't hurt. Will be one less thing to eliminate.

cruiser54

Actually, it's way more likely it's the tCU......

67 GMC

I agree. If there's no difference with the TCU connected or not, and all wiring is good to TCU, sounds like it could be bad. You can pick them up pretty cheap at junk yard. Online sources seem to charge a lot ($100 or more). I bought an 88 driveable parts-jeep for under $500 so would never pay that for a single part!

PapaBearXJ88

Thanks, that's what I was thinking, that's next - new TPS installed with no change. I'm checking the local junk yards now for a TCU. Would love to do the parts Jeep thing, but two problems - no room to store it and no $500.

Thanks again to you and Cruiser54 for the help. Did his grounding refresh tip and connector refreshing tip, that thing could be any better ground now unless I bury it!

67 GMC

You won't regret fixing grounds in a RENIX. Takes care of a lot of intermittent issues and ghost problems that start when the engine is hot but go away when it's cooled down.

PapaBearXJ88

Quick Update - I replaced the TCU, I THINK this might have been the issue, but I can not fully test it until I get the pinion gear replaced (shredded, big chunk busted off of it)
It feels like it is going into first gear when I put in drive to move it next to the shop, but won't know for sure until it's back running. Will update then.

PapaBearXJ88

I finally got my rear-end back together and was able to ops check whether the the TCU swap fixed the problem.

I swapped the TCU and BAM! - problem solved. I guess every now and then they go out, thank for all of your help, hope this thread helps someone else that has tranny electrical problems - since I had to go piece by piece all the way back to the TCU.